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Create sounds using analogue electronics (Part 7)

Posted: 27 Jul 2012     Print Version  Bookmark and Share

Keywords:Pitch-bend  voltage control  subtractive synthesis 

Because having separate filters for the oscillators enables them to be used as components of the final sound, rather than as a single source processed through a single modifier, this paralleling of facilities can be much more flexible in its creative possibilities. It is often used in formant synthesis, where the aim is to emulate the peaks in frequency response which characterise many real-world instruments, and particularly the human voice. Additive synthesis is an extension of this formant synthesis technique, where additional VCOs, VCFs and VCAs are added as required. Ganging of EGs by using voltage control of the EG parameters can make the control easier.

By using one VCO to modulate another, FM synthesis can be used, although the limitations of the VCO tuning stability and scaling accuracy limit its use. By using VCFs to process the outputs of each VCO, the FM can be dynamically changed from using sine waves to using more complex waveshapes by increasing the cut-off frequency of the VCF on the output of the modulation VCO. This is something which most commercial digital FM synthesisers cannot do!

The basic synthesiser patch varies between monophonic and polyphonic synthesisers. It is often simplified for use in polyphonic synthesisers: only one VCO and VCA, and often less controllable parameters. Custom 'synth-ona- chip' ICs are often used to implement polyphonic synthesiser designs, and these chips are based on a minimalist approach to the provision of modules and parameters.

Monophonic synthesisers
Monophonic synthesisers tend to be performance-oriented instruments designed for playing melodies, solos or lead lines. Despite the name, many monophonic analogue instruments can actually play more than one note at once: many have a duophonic note memory that allows two different note pitches to be assigned to two VCOs. With only one or two notes capable of being played simultaneously, an assignment strategy is required so that any additional notes played can be dealt with in a predictable way.

Two common schemes are last-note and low-note priorities. Last-note priority is a time-based scheme, which always assigns the most recently played note to the synthesiser's voice circuitry, whilst low-note priority is a pitch-based scheme, which always assigns the lowest pitched note to the voice circuitry.

Low-note priority can be a powerful performance feature; for example, the performer can play legato 'drone' notes with the thumb of their right hand and use the rest of the fingers to play runs on top, with staccato playing dropping back to the 'drone' note. This technique is most effective with envelopes that are not retriggerable; that is, they do not restart the attack segment each time a new key is pressed on the keyboard.

Portamento is a gliding effect which happens between notes. On a monophonic synthesiser it is normally used as a performance effect to give a contrast between the sudden pitch transition between notes and the slow change of a portamento. The portamento circuits in analogue synthesisers work by restricting the rate at which a CV can change. Normally, the pitch CV from a keyboard will change rapidly when a new note is selected. A portamento circuit changes the slope of the transition between the two voltages. It thus takes time for the note to move from the existing pitch to the new pitch (figure 2).

Figure 2: Portamento provides a smooth transition between successive pitches from the VCOs. The time taken for the keyboard CV to change from the previous value to the new value is called the portamento time.

Glissando is a rapid movement from one note to another where the pitch changes chromatically through all the notes in between. At fast speeds, glissandos sound similar to portamento.

Monophonic synthesisers normally arrange the front panel controls so that they form a logical arrangement, often mimicking the topology of the modules inside. The front panel is normally arranged so that sources and controllers are on the left, with modifiers and the final output on the right.

Early analogue monophonic synthesisers, and most modular systems, do not have any form of memory for the positions and settings of the front panel controls, and so a clear and functional arrangement of controls can aid the user in remembering settings. The process of using such a synthesiser requires a lot of practice to become thoroughly familiar with the workings of the instrument.

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